Sleep
I can set myself to awake awfully early in the morning, and it very often makes me awake very shortly before the time I had decided, usually a minute or less—and if it has ever failed, it can only be once or twice.
Also, it seems I can get by with less sleep than when using an alarm clock. I don’t know if this really is because your sleep cycle adjusts itself, but I’m less tired sleeping, on average, six hours per night, than when I sleep eight hours but abort my sleep by external means.
